1. Field of the Invention
The subject invention relates to lasers and, more particularly, to a method and apparatus employing stimulated Raman scattering for obtaining a larger number of Stokes shifted waves from a laser pump than heretofore possible.
2. Description of Related Art
Stimulated Raman scattering (SRS) was discovered at Hughes Aircraft Company in 1962. Until about 10 years ago, SRS was of scientific importance and was used as a tool for various spectroscopic tasks, but was not generally applied to the generation of new laser lines with substantial energy or power. At Hughes, use of Raman conversion was finally made for a laser cloud height indicator, and later for an eyesafe rangefinder (MELIOS). These systems used methane (CH.sub.4) as a vibrational Raman medium. The ordinary Nd:YAG line at 1.064 microns was shifted to 1.54 microns, which was considered eyesafe at the energy produced. Other commonly used Raman media are hydrogen (H.sub.2) and deuterium (D.sub.2).
For some applications, stimulated rotational Raman scattering (SRRS) has been considered more desirable than vibrational SRS. The rotational excitation of most molecules is of much lower energy than the vibrational excitation, and thus produces a smaller wavelength shift. Some prior art Stokes wave generators have used a single cell employing stimulated rotational Raman scattering (SRRS) to convert the pump wavelength into a series of longer wavelengths. Such prior art approaches to generation of Stokes shifted waves have typically exhibited two, or possibly three, Stokes lines in the output. Attempts to obtain more orders by increasing the intensity in such single cell system failed because of optical breakdown of the Raman medium.
Another device which has been used to generate Stokes shifted waves is an oscillator-amplifier configuration or master oscillator power amplifier (MOPA). Such an amplifier employs two cells. In a MOPA, a small portion of the pump is diverted into an oscillator cell, where a seed is created to drive an amplifier cell. Low power levels within the oscillator provide good seed beam quality. The seed from the oscillator is combined with the remainder of the pump energy inside the amplifier, where the stimulated emission process provides amplification of the seed, generating a high power Raman shifted output with good beam quality. Other experimenters have used this configuration to achieve high conversion efficiency into a single Stokes line. However, the configuration has not generated a large number of Stokes shifted waves.